The present invention relates to methods for the shaping of surfaces using lasers. In particular, the present invention relates to methods for the shaping of the cornea of the eye.
Many surgical procedures have been explored for reshaping the cornea. For example, to correct naturally occurring and postoperative astigmatism, procedures such as relaxing incisions (radial, transverse, arcuate), trapezoidal astigmatic keratectomy (Ruiz procedure), and wedge resection have been described. The number and the variety of techniques reflect the inability of any particular approach to provide adequate results in all patients (such as predictably eliminate corneal cylindrical error, in the case of astigmatism).
In a procedure known as radial keratotomy, radial incisions are made in the eye to remedy refractive errors such as myopia. The curvature of the eye is flattened when the incisions heal, thereby increasing the ocular focal distance.
The capacity of 193 nm argon-fluoride excimer laser to remove a precise amount of corneal tissue with submicron accuracy has let to clinical investigation of the excimer laser's ability to ablate corneal tissue so as to alter the curvature of the cornea for correction of refractive errors. This procedure is called photorefractive keratotomy (PRK).
The excimer laser can be utilized also to remove corneal opacities and other superficial corneal diseases in a procedure termed phototherapeutic keratotomy (PTK). PTK appears to be a useful and safe technique for treating corneal scarring from a variety of causes. However, approximately 50% of the patients experienced undesired hyperopic shifts secondary to corneal flattening from the procedure and, despite the use of surface modulators, elimination of pre-existing astigmatism was difficult.
Another potential use of the excimer laser is anterior lamellar keratectomy to smooth an irregular corneal surface. However, if an irregular surface is ablated without masking of the irregularities, the irregular surface contour will simply be reproductive deeper in the cornea.
U.S. Pat. Nos. 4,856,513 to Muller and 4,994,038 to Raven et al., the entire disclosures of which are incorporated by reference, disclose methods and apparatus for the shaping of surfaces using lasers. These patents are in particular directed to methods and apparatus which are described as particularly suited to the treatment of the cornea of an eye, providing a means of effecting reprofiling of the cornea.
U.S. Pat. No. 4,856,513 describes a laser system for reprofiling a surface, comprising a laser and an erodible mask disposed between the laser and the surface for providing a predefined profile of resistance to erosion by laser radiation. The masking means comprises a rigid structure which is affixed to the surface of the eye with a masking lens connected to the support structure and disposed above the cornea. The erodible mask is formed from plastic material, such as poly(methyl methacrylate) or poly(methyl styrene); alternatively, the rigid structure of the masking means may comprise a cup-shaped rim to support a liquid or semi-liquid masking lens. During irradiation, the lens is gradually ablated and an increasing area of the cornea becomes exposed to erosion; the maximum thickness of the lens exceeds the minimum thickness by an amount equal to the maximum depth of the erosion desired.
In addition to the afore-noted type of masking means, U.S. Pat. No. 4,994,038 further discloses an embodiment of masking means in the form of a contact-type lens device which is disposed upon and directly affixed to the surface of the cornea. The contact-type lens is constructed so as to have a first surface contoured to fit to the surface to be eroded and a second surface contoured to provide the desired surface contour following erosion by exposure to laser radiation. The contact-type lens comprises a material which is erodible by laser radiation and preferably has ablation characteristics substantially identical to the object material; once again, plastics material such as poly(methyl methacrylate) and poly(methyl styrene) are proposed.
While providing advantages relative to the direct application of the laser to the corneal surface, an entirely satisfactory method for reshaping the corneal surface has not been provided by using either a rigid structure affixed to the surface of the eye with a masking lens connected thereto (as proposed in U.S. Pat. No. 4,856,513) or a masking means in the form of a contact-type lens made of plastic (as proposed in U.S. Pat. No. 4,994,038). The rigid structure of U.S. Pat. No. 4,856,513 is a cumbersome device and would not be particularly suitable for routine use.
Moreover, both arrangements call for the use of erodible masks which are manufactured based upon measurements of the patient's eye. In principle, the prior art methods call for preformed masks which are configured to take into account both the structure of the surface to be eroded and the desired surface contour following erosion, so as to provide a predefined profile of resistance to erosion by laser irradiation. The preparation of such masks would thus require a complex series of measurements to define the surface to be treated and a manufacturing process which would permit the formation of highly-complex profiles on both surfaces of the mask. With respect to highly scarred or irregular corneas in particular, the preparation of such masks would be extremely difficult if not impossible. The preparation of masks from materials having sufficiently similar ablation characteristics to the surface being treated is also more easily accomplished in theory than in practice.
It is therefore an object of the present invention to provide a method whereby modifications of the cornea by laser treatment may be carried out in a more efficient manner than was heretofore possible with the prior art methods.
In particular, it is an object of the present invention to provide a method for reprofiling the surface of the cornea which does not involve the preparation of complex ablation masks or the use of cumbersome devices for maintaining such ablation masks in a fixed position.
It is a further object of the present invention to provide a method for reprofiling the surface of the cornea which utilizes readily-available materials, including masking materials which have ablation properties substantially identical to those of the cornea.